Method and apparatus for in-process liquid analysis by laser induced plasma spectroscopy

ABSTRACT

A method and apparatus for spectrochemical analysis of liquids, including molten metals, using laser-induced plasma spectroscopy. The apparatus preferably comprises a high power pulsed laser focused on the surface of a liquid stream flowing in a measurement cell, and an optical spectrometer-detector assembly, which receives, detects and analyzes the radiation emitted by the high temperature plasma thereby excited. The measurement cell, and optional pump, establish laminar flow of the liquid flow, thereby permitting the laser to repeatedly sample a fresh un-perturbed surface, while also ensuring that bubbles formed in the liquid are removed from the focal volume Preferably, a blower prevents aerosols and matter ejected from the sample responsive to the incident energy from interacting with subsequent laser pulses, and from accumulating on the optic.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an apparatus and methods for use in laserinduced breakdown spectroscopy (LIBS), and for the rapid analysis ofliquids including molten metals. In particular, the invention isdirected to an apparatus and methods for use with LIBS system that canbe applied to the real time analysis of a flowing liquid, and overcomesaccuracy problems that are associated with LIBS induced aerosolsincluding the accumulation of droplets on the laser optics.

2. Related Art

Due to the absence of suitable on-line liquid analysis technology, thereare many instances where industrial processes must be monitored byperiodic liquid sampling followed by time consuming laboratoryprocedures, such as liquid or gas chromatography, graphite furnaceatomic absorption spectroscopy, or inductively coupled plasma opticalemission spectrometry. Faster in-situ methods such as spark-dischargeoptical spectrometry are only applicable to electrically conductivematerials, while X-ray backscattering probes are limited in sensitivity.

Laser induced breakdown spectroscopy can provide rapid, in-situcompositional analysis of a variety of materials in hostileenvironments, and at a distance. This method includes focusing a highpower pulsed laser on the material, thereby vaporizing and ionizing asmall volume of the material to produce a plasma having an elementalcomposition that is representative of the material. The optical emissionof the plasma is analyzed with an optical spectrometer to obtain itsatomic composition.

A method for analyzing elements present in a sample using LIBS is knownin the art. For example, a list of patents that are related can be foundin U.S. Pat. No. 5,751,416, which is incorporated herein by reference.Furthermore this method has been applied to a variety of materials andindustrial environments, as exemplified in the following documents thatare related to the analysis of liquids.

U.S. Pat. No. 4,986,658, incorporated herein by reference, describes aprobe for performing molten metal analysis by laser induced plasmaspectroscopy. The probe contains a high-power laser that produces apulse that has a triangular pulse waveshape. When the probe head isimmersed in molten metal, the pulsed laser beam vaporizes a portion ofthe molten metal to produce plasma having an elemental composition thatis representative of the molten metal composition. Within the probethere is provided a pair of spectrographs, with each having adiffraction grating coupled to a gated intensified photodiode array. Thespectroscopic atomic emission of the plasma is detected and analyzed fortwo separate time windows during the life of the plasma by using twospectrometers in parallel. The first time window analyzes the plasmaplume before it reaches thermal equilibrium shortly after thetermination of the laser pulse (typically 10 nanoseconds-long) to detectline reversals, as caused by the absorption of radiation emitted by thehotter inner portion of the plasma plume by relatively cooler outerportions of the plasma plume. Once the plasma has reached thermalequilibrium, typically 1 microsecond later, a second time windowanalyzes the more conventional line emissions from the opticallyemissive plasma. The spectra obtained during either the first or thesecond time window, or a combination of both, can be used to infer theatomic composition of the molten metal. In this configuration forobtaining an elemental composition that is representative of the liquid,the probe head must be immersed in the liquid or the molten metal.However, the immersed probe system is not easy to use and is notsuitable for use with most molten metals or melt glass.

U.S. Pat. No. 5,379,103, incorporated herein by reference, describes amobile laboratory for in-situ detection of organic and heavy metalpollutants in ground water. Pulsed laser energy is delivered by fiberoptics to create a laser spark on a remotely located analysis sample.The system operates in two modes, one is based on laser induced plasmaspectroscopy, and the other on laser induced fluorescence. In the firstoperational mode, the laser beam emerging from the fiber optics isfocused on the sample by a lens to generate a plasma. The emittedspectrum is analyzed and used to detect heavy metals. In the second modean un-focused ultraviolet laser beam from the fiber optics irradiatesthe sample, thereby exciting fluorescence from organic molecules with anaromatic structure. The emitted fluorescence is transmitted via fiberoptics for further analysis. The measured spectral and temporalcharacteristics of the emitted fluorescence can then be compared withpredetermined characteristics to identify the organic substances in theanalysis sample. Again, in this patent laser pulses are used to analyzeon-site pollutants in stationary ground water. This approach does notprovide any arrangement related to the real time analysis of a liquidstream or propose solutions to problems associated with the presentinvention.

Two temporally close sparks induced by two collinear lasers are used inU.S. Pat. No. 4,925,307, incorporated herein by reference, for thespectrochemical analysis of liquids. The laser light is notsignificantly absorbed by the sample so that the sparks occur in thevolume inside the liquid. The spark produced by the first laser pulseproduces a bubble in the liquid that stays in the gaseous state forhundreds of microseconds after the first spark has decayed, so that thesecond laser pulse, fired typically 18 microseconds after the firstpulse, will produce a second spark within the gaseous bubble. Theemission spectrum of the second spark, detected by a spectrometeroriented at 90 degrees from the laser beam axis, is thus much moreintense and exhibits reduced line widths compared to the first spark, sothat an increased detectability of the atomic species is obtained bysampling the bubble with the second laser spark. This approach can notbe used for molten metals, opaque liquids or for real time measurement,as it is only suitable for off-line analysis of relatively transparentliquids.

As mentioned above, the use of laser induced plasma spectroscopy foranalysis of liquids is known. In particular, three approaches have beendescribed. The first approach, as used by Wachter and Cremers (AppliedSpectroscopy, Vol 41(6), 1042-1048, 1987), Arca et al (AppliedSpectroscopy, Vol 51(8), 1102-1105, 1997) and Berman et al (AppliedSpectroscopy, Vol 52(3), 438-443, 1998), consists of focusing laserpulses onto the surface of a stationary liquid body under laboratoryconditions. This approach is not useful for on-line measurement.

The second approach, as described by Ng et al (Applied Spectroscopy, Vol51(7), 976-983, 1997) and Ho et al (Applied Spectroscopy, Vol 51(1),87-91, 1997), is devoted to the analysis of liquids, which are ejectedthrough narrow tubing to form a vertical jet. The jet is intercepted byan ablation laser about 12 mm downstream. No mention is made of theanalysis of a controlled liquid laminar flow.

The last approach, as adopted by Winefordener et al (Analytica ChimicaActa, Vol 269(1), 123-128, 1992), concerns the analysis of liquidaerosol. The liquid aerosol was generated with a commonly usedInductively Coupled Plasma-type glass concentric nebulizer assembly, andcarried by the nebulization argon flow (0.5 l min-1) through a smalltube (1 mm diameter) into a laser induced plasma sustained in ambientlaboratory air. This approach is not adequate for on-line measurement.

SUMMARY OF THE INVENTION

Briefly, the technique of the present invention is to monitor variouselements in liquids, including molten metal, during normal processingoperations, preferably while the liquid is flowing, as opposed toremoving a sample from the liquid stream for laboratory analysis. Directmonitoring of the flowing liquid provides many advantages over discretesampling, including the ability to adjust the process being monitored inreal time based on the results of the analysis. However, the inventorshave found that frequent cleaning of the optical component (focusinglens or window) may be required due to the absorption of laser andemitted light by accumulated matter that was ejected and splashed fromthe liquid sample in response to the incident laser pulses. Moreover,vaporization of the liquid sample during the detection and analysisprocess creates miniature shock waves that create aerosols in the volumeabove the liquid surface and waves on that surface. As a result, theoverall efficiency of the direct monitoring process may be affected.Furthermore, it appears that laser pulses may induce bubbles inside someliquids that are transparent at the laser wavelength. These bubbles mayreach the surface being analyzed and change the characteristics of thelaser-induced plasma, thereby affecting measurement reproducibility.

In view of the above, the object of the present invention is to providea method and apparatus which permits the reliable analysis of a laminarflow of liquid by focusing laser pulses on the surface of that liquid.Also, the invention provides a means for direct monitoring of a liquidstream with a LIBS system, while overcoming the problems associated withaerosols, waves, debris, or droplets on the focusing lens, therebyachieving efficient continuous LIBS analysis. The present invention bothenables the laser to repeatedly sample a fresh surface, and largelyprevents aerosols and matter ejected from the sample responsive to theincident energy from accumulating on the optics and absorbing the laserlight. Furthermore, circulation of the liquid flow removes bubbles fromthe focal volume, and thereby prevents them from reaching the surfacesampled by the laser pulse where they would interfere with measurements.

Accordingly, one object of this invention is to provide an improvedmethod and apparatus for in-situ transient spectroscopic analysis ofliquids including molten metal.

A further object of this invention is to provide an apparatus thatfacilitates reliable real time LIBS analysis by establishing a laminarflow of liquid, thereby enabling the laser to sample a fresh stablesurface, preventing bubbles formed in the liquid from reaching thesampled surface by moving them away from the focal volume, and removingwaves generated on the surface by the laser to prevent them fromperturbing the surface during subsequent measurements.

It is a further object of some aspects of the present invention toprovide means for preventing aerosols and matter ejected from the sampleresponsive to the incident energy from accumulating on the optic andabsorbing both the laser beam and plasma radiation entering thespectrometer optics.

It is still a further object of the present invention to provide animproved optical assembly for use in a variety of industrialenvironments.

According to one aspect of the present invention an apparatus isprovided for the optical analysis of the concentrations of one or moreelements in a liquid, by laser-induced plasma spectroscopic analysis.The apparatus comprises a means for emitting and focusing laser pulseson a surface of the liquid to generate a plasma that emits opticalradiation that contains elemental radiation that is derived fromseparate compositional elements of the liquid; a detector; an airremoval exhaust or blower to substantially prevent drops, which areejected from the liquid in response to the incident energy, fromaccumulating on an optical window of said optical system; and a cellassembly, which establishes a substantially laminar flow of the liquidto be analyzed, including a controller to control the liquid surfacelevel and speed of flow.

According to other aspects of the present invention the detectorcomprises a collinear (or non-collinear) optical system and spectrometerfor sampling and measuring the radiation spectrum, including thespecific line emissions that are representative of selected elementspresent in the liquid; and data processing means for determining theconcentration of the selected elements by comparison with formerlyestablished calibration curves obtained by using standard(predetermined), precalibrated samples with different elementalconcentrations independently measured by established laboratorytechniques.

According to other aspects of the present invention the detectorcomprises a photodiode array, CCD camera, or photomultipliersindividually positioned to detect both emissions from elements presentin the liquid and background radiation.

According to another aspect of the present invention the means foremitting and focusing laser pulses uses double laser pulses for creatingand exciting plasma from the liquid.

According to another aspect of the present invention the apparatusfurther comprises fiber optics means to convey radiation emitted by theplasma to the spectrometer.

According to another aspect of the present invention the cell assemblyof the apparatus is connected to a pump that allows for circulation ofthe liquid in a closed loop.

According to another aspect of the present invention there is a methodfor optically analyzing the concentrations of one or more elements in aliquid, by laser-induced plasma spectroscopic analysis, comprising thesteps of emitting and focusing laser pulses on a surface of the liquidto generate a plasma that emits optical radiation that containselemental radiation that is derived from separate compositional elementsof the liquid; detecting the optical radiation; removing or blowing airto substantially prevent drops, which are ejected from the liquid inresponse to the incident energy, from accumulating on an optical windowof said optical system; and establishing a substantially laminar flow ofthe liquid to be analyzed by using a controller to control the liquidsurface level and speed of flow.

According to another aspect of the present invention the detecting stepincludes sampling and measuring the radiation spectrum, including thespecific line emissions that are representative of selected elementspresent in the liquid using a collinear (or non-collinear) opticalsystem and spectrometer; and processing the data to determine theconcentration of the selected elements by comparing them with formerlyestablished calibration curves that were obtained by recording thenormalized signal levels corresponding to samples with differentelemental concentrations independently measured by establishedlaboratory techniques.

According to other aspects of the present invention the detecting stepincludes using a photodiode array, CCD camera, or photomultipliersindividually positioned to detect both emissions from elements presentin the liquid and background radiation.

According to another aspect of the present invention the emitting andfocusing laser pulses step uses double laser pulses for creating andexciting plasma from the liquid.

According to another aspect of the present invention the method furthercomprises the step of conveying radiation emitted by the plasma to thespectrometer in said detecting step using fiber optics.

According to yet another aspect of the present invention the methodfurther comprises the step of circulating the liquid in a closed loop byusing a pump.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects, features and advantages of the presentinvention will become apparent from a consideration of the followingdetailed description of the invention in conjunction with the drawingfigures in which:

FIG. 1 is an overall block diagram of the apparatus.

FIG. 2 shows a detailed set-up for the flow cell.

FIG. 3 shows a spectrum obtained with a Nd:YAG Q-switched laser focusedon the surface of tap water in the cell showing the calcium lines at393.3 and 396.8 nm. The laser energy was 200 mJ.

FIG. 4 shows the calibration curve for calcium in water.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The method and apparatus of this invention use powerful laser pulses toirradiate an unknown liquid, and thereby form microplasma or a spark atits surface. As a result of the high temperature thus generated, aminute amount of the material is ablated, thereby producing a smallcloud of excited atoms and ions which subsequently decay emittingcharacteristic radiation from which they may be identified by spectraland temporal resolution of the emitted light. The concentration of aparticular element in the liquid can be found by comparing the emittedradiation with predetermined calibration curves that were obtained byrecording the normalized signal levels corresponding to samples withdifferent elemental concentrations independently measured by establishedlaboratory techniques.

FIGS. 1 and 2 respectively show a block diagram and schematic diagram ofthe apparatus according to the present invention. The individualcomponents shown in outline or designated by blocks in these figures areall well-known in the LIBS arts, and their specific construction andoperation are not critical to the operation or best mode for carryingout the present invention. The probe 10 includes a first mirror 12 thatreflects a laser pulse (generated by the laser source head 14) by ninetydegrees to a focusing lens 13. The second (dichroic) mirror 16 reflectsthe laser pulse at ninety degrees to the surface of liquid point (A)located in the bath of the flow cell 5. Spectral response signalsgenerated by plasma created at the liquid surface (A) by the applicationof the laser pulse to the liquid surface are focused by a second lens 18at the entrance of fiber optic 20. The light is guided by fiber optic tothe spectrometer 22, which will be described in more detail later.Detection signals generated by a photodiode array or a CCD camera or PMs(photomultipliers) of a detection portion 7 of the spectrometer aresupplied to a delay generator of the computer control-processing unit 24for processing and treatment of data, and for the evaluation of the datato determine the concentration of various elements within the liquid. Ablower 26 is located just above the flow cell 5. It prevents the debris,particles, or drops of liquid generated by the laser pulse focused onthe sample from reaching the quartz window 28 by blowing airperpendicular to the laser beam. It also clears aerosols formed by thelaser pulse from the path of the laser beam, which enables laser pulsesto reach the sample without being absorbed by these aerosols.

FIG. 2 shows a more detailed lateral view of the flow cell. Asschematically illustrated in this figure, the reservoir 2 is fed by aflow of liquid coming from the pump 31 or from the liquid stream of theprocess to be analyzed. The velocity of the liquid in the tube 3 ismaintained constant by keeping the height of the liquid in the reservoir2 at the level of the evacuation tube 4. When the feeding flow of thepump or the liquid stream exceeds the flow of the tube 3, the tube 4will evacuate the extra flow. As a result, the heights of the liquid inthe reservoir 2 and cell 5 are maintained substantially constant, whichensures a uniform flow through the cell 5 and a stable surface. Thespeed and surface position of the liquid can be controlled by the heightof the reservoir 2 above the cell 5, the flow control valve 39, theheight of the weir 40, and the cross-section area of the bath 5. Theflow cell operates in two modes, depending on the feeding flow from thepump or the process; the closed mode is used to calibrate the systemwhile the open mode is devoted to on-line measurement of the liquidstream to be analyzed. To promote uniform flow of the liquid and ensurethat its surface is representative of the bulk, a mixing screen 8 may beplaced at the entrance of the bath.

EXAMPLE 1

In its preferred embodiment, the present invention comprises anapparatus for LIBS analysis comprising a liquid sample cell and feedarrangement for presenting a substantially uniformly flowing liquid atconstant level to a pulsed laser focused on the liquid surface, meansfor conveying radiation emitted by the thus excited plasma to aspectrometer, and means for detecting and analyzing radiationcharacteristic of elements present in the liquid.

In example 1, liquid (such as water with a low concentration of Calcium)enters the end of the 12 inch long, 2 inch deep and ¾ inch wide highdensity polyethylene sample cell (or bath) 5 via a flow control valve 39and half inch diameter pipe 3, and the liquid is transformed into auniform flow by means of a PVC mixing plate 8 perforated with ⅛ inchdiameter holes on {fraction (7/32)} inch staggered centers and a weir40. Constant liquid depth is maintained at 1.75 inch by a constantfeeding head, and a weir 40 at the outflow end of the cell. A flow rateof between 1 and 1.5 l/min thus produced is sufficient to prevent anybubbles (by sweeping the bubbles from the analyzed region of the liquid)generated by laser pulses from interfering with subsequent measurementsperformed at a 1 Hz pulse repetition rate. A blower 26, such as the 26CFM flatpak EBM Pabst model RL 90-18-00, positioned to blow air paralleland close to the liquid surface, deflects splashed or otherwise ejectedliquid, thus substantially reducing contamination of a 2 inch diameteroptical window 28 positioned about 12 inches above the liquid. Four inchducting 29 is used to channel air to the blower, and, via a collectionfunnel 33, to remove it and ejected material, from the apparatus. Anintake filter may be used to remove suspended particles from the airwhen such particles may trigger unwanted plasma emissions. A cell coverplate 30 with a ¾ inch hole centered on the laser beam near the centerof the cell serves to shield the liquid from disturbance by the blower.

A suitable choice of laser with sufficient power to excite plasma withradiation characteristic of the composition of the liquid is the Big SkyModel CFR 400 Nd:YAG 400 mJ NIR laser, in combination with a 40 cm focallength focusing lens.

Optical emission from the plasma passes through a protective window 28that is substantially collinear with the laser beam. The emission isseparated from the path of the laser beam by a dichroic mirror andfocused by a lens into optical fibers, whereby it is conveyed foranalysis to an optical spectrometer. A 0.35 m Czerny-Turner spectrometer22 with a 50 micron slit width and a 3600 grooves/mm grating may be usedin conjunction with a gated intensified CCD camera 7, manufactured byAndor Technology. Alternatively, a photodiode array detector, orphotomultipliers individually positioned to detect both emissions fromelements present in the liquid and background radiation, may provideuseful measurements. Selection of spectral peaks to be measured dependson the application. For the analysis of calcium in water, the ionicemission peak at 393.3 nm yields a linear calibration from 0 ppm to atleast 500 ppm, using an acquisition delay of 1 microsecond andintegration time of 5 microseconds. Continuum emission at a nearbywavelength with no spectral emission serves to normalize themeasurement.

In some preferred embodiments of the present invention, the flow cell isconnected to a pump 31 that allows circulation of liquid from areservoir 32 at a set velocity controlled by the level of liquidmaintained in a reservoir 2 feeding the cell. Here valves 34, 35 and 36are closed, and valves 37 and 38 opened, to permit liquid to be pumpedin a closed loop for calibration and other purposes. In anotherpreferred embodiment, a blower or exhaust air removal is added to thecell to clear the laser beam path of aerosols to thereby prevent thewindow from accumulating debris and small drops of liquid splashed bythe laser pulse.

An example of the spectrum obtained with such an apparatus is shown inFIG. 3. The spectrum was obtained from an approximately 1 mm-diameterspot at the surface of water containing 50 ppm, of Ca by firing a singlelaser pulse shot of 200 mJ energy provided by a YAG laser at awavelength of 1064 nm.

Results

Table 1 shows a comparison of the measurement reproducibility obtainedby focusing laser pulses on the surface of water with and without an airremoval exhaust or blower. For quantitative analysis by laser-inducedplasma spectroscopy, elements are monitored by the measurement of theirspectral line intensities, which are proportional to the speciesconcentrations. These line intensities are affected by severalparameters. In particular, they are highly dependant on the amounts ofvaporization and the degree of ionization, which can change as afunction of laser wavelength, laser fluence, pulse-to-pulse variability,sample surface morphology, ambient gas pressure, and ambient gasspecies. When the bubbles created inside the liquid by the laser pulseburst at the surface, they change the angle of incidence between thelaser beam at the liquid surface, which, in turn, can change the fluenceof the laser, and the line intensity. Also aerosols created by thelaser-liquid interaction absorb the laser beam and prevent partially thelaser from reaching the surface of the sample. This absorption canchange the reproducibility of the measurement by affecting the energydelivered to the sample. The results obtained in Table 1 show clearlythe power of our invention, how the aerosols affect the results and howthe use of the blower improves the accuracy of the measurements and makeit possible to realize quantitative measurements.

TABLE 1 Comparison of the reproducibility of single shot lasermeasurements with and without air blowing and liquid motion NetAmplitude Liquid Blower Ca 393.3 nm Std. Dev'n SD/Amp % Flowing On 96302631 27.3 Flowing Off 5715 3314 58.0 Stationary Off 5180 4037 77.9Stationary On 9883 3439 34.9

Table 1 also compares results obtained for the Ca 393.3 nm line withflowing and stationary water (moving and non-moving surfaces). Hereagain measurement accuracy is improved by our set-up. Moreover thisarrangement lends itself to the analysis of continuously flowingliquids. The calibration curve obtained with the system on watercontaining different Ca concentrations is shown in FIG. 4. One canappreciate the good linearity and reproducibility of the calibrationcurve obtained with this approach. Reproducibility for measurementsbased on 100 laser shots is better than 3%.

This invention may be applied in a number of industrial processes.Dissolved arsenic in aqueous acidic solution by-products from coppersmelting requires costly treatment to precipitate the arsenic prior tosolution neutralization and disposal in an environmentally acceptablemanner. Continuous arsenic analysis permits substantial cost savingsthrough more accurate matching of reagent use to arsenic content. Otherapplications include in-process monitoring and control, as well asoptimization of toxic effluent treatments in the electro-refining ofcopper. Aqueous effluent monitoring in mining and other industrialsettings, as well as non-industrial effluent monitoring, are other usesfor the present invention.

The present invention may also be applied in the pharmaceutical andother industries for both aqueous and non-aqueous liquids. In thepharmaceutical industry, for example, the invention may be used invarious ways for the analysis of liquid products. One possibility is toexploit certain elements that are uniquely associated with activeagents, as opposed to other components of the formulation, as indicatorsof concentration. Such elements may, for example, be phosphorus, sodium,sulfur, or sodium—all of which lend themselves to LIBS analysis which,using the present invention, may be performed in under 1 minute. Thealternative of off-line determination by liquid chromatography requiressample preparation and analysis that take one or more hours.

Conclusion

Thus, what has been described is an improved method and apparatus forin-situ transient spectroscopic analysis of liquid. While the presentinvention has been described with respect to what is presentlyconsidered to be the preferred embodiment, it is to be understood thatthe invention is not limited to the disclosed embodiment. To thecontrary, the invention is intended to cover various modifications andequivalent arrangements included within the spirit and scope of theappended claims. Therefore, the scope of the following claims is to beaccorded the broadest interpretation so as to encompass all suchmodifications and equivalents

What is claimed is:
 1. A laser-induced plasma spectroscopic apparatusfor analysing a liquid comprising: an optical structure to direct a beamof laser pulses to a surface of the liquid to generate a plasma thatemits elemental radiation that corresponds to at least one compositionalelement of the liquid; a detector structure for detecting said emittedelemental radiation; and a cell assembly which establishes asubstantially laminar flow of the liquid past the detector structure,said laminar flow sufficient to prevent said beam of laser pulses fromperturbing a region on said surface within a focal volume of saiddetector structure, said cell assembly including a controller to controla liquid surface level.
 2. An apparatus according to claim 1, whereinthe controller controls a liquid speed of flow.
 3. A method oflaser-induced plasma spectroscopic analysis of a liquid, comprising thesteps of: establishing a substantially laminar flow of the liquid;directing a beam of laser pulses to a surface of the liquid to generatea plasma that emits elemental radiation that corresponds to at least onecompositional element of the liquid; and detecting the emitted elementalradiation using a detector; wherein said laminar flow is sufficient toprevent said beam of laser pulses from perturbing a region on saidsurface within a focal volume of said detector.
 4. A method according toclaim 3, wherein said establishing step includes controlling a liquidspeed of flow.
 5. A method according to claim 3, wherein said detectingstep uses an optical window and said establishing step includesestablishing a substantially laminar flow of the liquid past saidoptical window and controlling a liquid surface level.
 6. A methodaccording to claim 5 further comprising generating an air flow betweensaid optical window and the liquid, said air flow substantiallypreventing liquid drops, which are ejected from the liquid in responseto said beam of lacer pulses, from accumulating on said optical window.7. The laser-induced plasma spectroscopic apparatus of claim 1 whereinsaid laminar flow is sufficient to prevent waves being induced withinsaid region on said surface by said beam of laser pulses.
 8. Thelaser-induced plasma spectroscopic apparatus of claim 1 wherein saidlaminar flow is sufficient to prevent bubbles induced in the liquid bysaid beam of laser pulses from reaching said surface within said region.9. The method of claim 3, wherein said laminar flow is sufficient toprevent waves being induced within said region on said surface by saidbeam of laser pulses.
 10. The method of claim 3 wherein said laminarflow is sufficient to prevent bubbles induced in the liquid by said beamof laser pulses from reaching said surface within said region.